1. Field of the Invention
The present invention relates generally to a method for manufacturing an array of nanostructures and a substrate with an ordered array of nanostructures, wherein the nanostructure size is controlled, and to a substrate including an ordered array of nanoclusters. More particularly, the method comprises etching structures into a substrate and/or depositing adatoms onto the substrate through a patterned mask and an intermediate transfer layer between the mask and the substrate.
2. Description of Related Art
The ability to control function by controlling size makes nanoclusters very attractive for technological applications in high-speed computing, high density data storage and display, and optical communications through devices such as the single-electron transistor and the quantum dot laser. Designs for such devices require not only sharp control of nanocluster size, but also fabrication of ordered arrays of nanoclusters and, in some cases, interconnections between clusters within the array.
As has been discussed elsewhere [for example, J.-M. Gerard (1995)], single layer quantum dot arrays have been demonstrated to have excellent optical properties such as high quantum efficiency, long radiative lifetimes, and very fast PL rise times. However, direct growth has been stymied by xe2x80x9cthe prerequisite of an ultrafine lithographic definition of the mask.xe2x80x9d
Dramatic advances have been made recently in obtaining ordered arrays of nanoclusters from liquid phase syntheses by selective precipitation and Langmuir-Blodgett techniques [Murray et al. (1993); Ohara et al. (1995); Murray et al. (1995); Whetten et al. (1996); Luedtke et al. (1996); Heath et al. (1997)]. Ordered arrays have also been produced using films of close-packed polystyrene spheres as deposition masks [Hulteen et al. (1995)]. Ensembles of individual, size-controlled InP quantum dots grown by self-assembly in molecular beam epitaxy on a GaAs surface have emitted light of very narrow bandwidth at a wavelength determined by the size of the dots [Grundmann et al. (1995)]; embedded between electron-injecting and hole-injecting layers, these dots have exhibited lasing [Kirstaedter (1996)]. However, because they grow at randomly distributed nucleation sites on the substrate, their location is difficult to control.
From the point of view of device fabrication, it is desirable to first define the desired nanoscale array pattern directly on the substrate and then grow or deposit the nanoclusters on the patterned substrate. The nanoclusters produced preferably have diameters less than about 25 nm to show true quantum confinement behavior.
In earlier work, Heath and co-workers studied the formation of clusters in confined geometries by defining 100 and 150 nm diameter holes in a thin oxide mask over a Si wafer and then growing Ge clusters on the Si surfaces exposed in the etched holes [Heath et al. (1996)]. They observed a few clusters in each 150 nm hole at locations distributed over the bottom of the hole. A few of the 100 nm holes contained a single cluster, but difficulties with that sample precluded complete analysis. Their results showed that the confining geometry of the 150 nm hole limited the number and size of clusters growing in the hole but did not precisely control their location.
To manufacture functional devices incorporating nanostructures, it is important to obtain a precise spatial arrangement of the quantum dot array and a uniform size and composition of the individual dots. However, two-dimensional arrays of quantum dots fabricated by the self-assembled growth of strained islands of materials using the Stranski-Krastanow growth mode typically exhibit a randomness of spatial nucleation [Huffaker et al. (1998); Heinrichdorff et al. (1997)]. Recently, studies using pre-patterned surfaces formed by nanoimprinting have been shown to promote improvement in the spatial uniformity of dot nucleation [Kamins et al. (1999)]. Also, selective area growth employing block copolymer lithography has achieved a high degree of uniformity in the spatial position of dense arrays of nanoscale dots with the diameter of the dots estimated to be 23 nm [Li et al. (2000)]. The dots are hexagonally close packed and are organized into domains roughly 1 xcexcm in size.
Nevertheless, smaller dots are desirable for use in quantum dot devices. Further, there remains a need for a method of forming nanostructures on a variety of substrates having differing surface properties.
Thus, there is a need for a method for transferring nanoscale patterns to a wider variety of substrates to facilitate manufacture of a wider range of quantum dot type devices.
It is an object of the present invention to provide a method for applying nanopatterns to substrates having greater surface roughnesses than has hitherto been possible.
It is another object of the present invention to provide a method for applying nanopatterns to substrates with intrinsic surface chemistries that are inappropriate for the successful application of nanomasks directly to the surface to be patterned.
It is a yet another object of the present invention to provide a method for using nanomasks to apply nanopatterns to substrates without roughening, degrading, or otherwise damaging the newly nanopatterned surface.
It is still another object of the present invention to provide a method for removing nanomasks after application of nanopatterns to substrates without roughening, degrading, or otherwise damaging the newly nanopatterned surface.
It is a further object of the present invention to provide a method for transferring a nanoscale pattern to a substrate via dry etching when there is insufficient etch selectivity between the mask and the surface to be patterned.
It is yet a further object of the present invention to create arrays of nanostructures on a substrate surface by replicating the inverse of a nanomask pattern onto the substrate surface without first creating wells or holes in the substrate.
One embodiment of the present invention comprises a method for producing an ordered array of nanostructures on a substrate surface. The method includes the steps of providing a substrate; forming an intermediate transfer layer on the substrate surface; mounting a mask template on the intermediate transfer layer; and etching a pattern through the intermediate transfer layer, wherein the pattern substantially replicates the size and spacing of the ordered array of nanoscale holes in the mask template. The mask template includes an ordered array of nanoscale pores therein, and the etching step may comprise forming an array of wells in the substrate.
The method may include an additional step of applying a coating to the mask template to form a patterning mask. Other optional steps include removing the mask template from the intermediate transfer layer, modifying the surface properties of the intermediate transfer layer prior to the mounting step, and depositing adatoms onto the substrate surface through the pattern of wells in the intermediate transfer layer. There may also be a step of modifying the substrate surface where it is exposed by the etching step. Adatoms may be deposited or adsorbed onto the modified substrate surface, either prior to or after removal of the intermediate transfer layer.
In accordance with the method for producing the ordered array of nanostructures on the substrate surface, the substrate may comprise a material having a surface roughness greater than about 1 nm RMS roughness. The mask template may comprise a material selected from inorganic crystalline materials, glass, block copolymers, naturally occurring biological materials, modified biological materials, and biomimetically grown materials. The intermediate transfer layer may comprise a self-developing resist, such as formvar, polyimide, photoresists, PMMA nitrocellulose, ammonium perchlorate, oxalic acid, or picric acid.
Another embodiment of the present invention comprises a method for producing an ordered array of nanostructures on a substrate surface. The method includes the steps of providing a substrate, forming an intermediate transfer layer on the substrate, mounting a mask template on the intermediate transfer layer, transferring a first pattern comprising an ordered array of nanostructures to the intermediate transfer layer, and transferring a second pattern to the substrate. The first pattern may be transferred to the intermediate transfer layer by deposition of adatoms onto the intermediate transfer layer, etching wells into the intermediate transfer layer, and combinations thereof. The second pattern may be transferred to the substrate by a method selected from dry etching, deposition of adatoms onto the substrate, and combinations thereof. In the method, the first pattern in the intermediate transfer layer includes an ordered array of nanostructures corresponding to nanoscale pores in the mask template, and the second pattern may comprise an ordered array of nanostructures corresponding to either the mask template pattern or the inverse of the mask template pattern.
Either of the pattern transfer steps may include dry etching, such as inductively coupled plasma etching. The use of the intermediate transfer layer may facilitate the use of materials for the mask template and the substrate that have similar etch rates to obtain etched features in the substrate.
Yet another embodiment of the present invention comprises a substrate having a surface and including an ordered array of nanostructures, wherein the substrate surface is hydrophobic, has a RMS roughness greater than about 1 nm RMS roughness, has an etch rate substantially similar to the etch rate of a patterning mask used to form the ordered array of nanostructures, and/or is substantially without wells for containing the nanostructures. The nanostructures may be selected from clusters of adatoms, wells, and combinations thereof. The features may have diameters between about 2 nm and about 10 nm, and the lattice spacing may be between about 3 nm and about 30 mn.